Fetuses as young as 16 weeks <a href="http://www.ncbi.nlm.nih.gov/pubmed/2192091" target="_blank">have been spotted with teensy protuberances</a> that rise and fall throughout gestation, according to studies in the Journal of Ultrasound Medicine. Scientists aren't sure why this happens, but one theory, reported in the Los Angeles Times, is <a href="http://www.latimes.com/features/health/la-he-mating18-2008aug18,0,4651977.story" target="_blank">that these periodic movements help the penile tissue stay oxygenated</a> and healthy.